Pressure-sensitive carbonless paper recording systems have evolved into a very large market, chiefly due to their convenience compared with carbon paper copying. A review of the state of the carbonless paper art is Petitpierre, "DYES AND CHEMICALS FOR CARBONLESS COPYING PAPER: HISTORY, PRESENT SITUATION, PROBLEMS AND TRENDS", 1983 Coating Conference, TAPPI PROCEEDINGS, p. 157-165, herein incorporated by reference. As described therein, carbonless paper involves a manifold comprising a matched pair of paper sheets termed a donating paper and a receiving paper. When written upon, the donor paper is pressed into contact with the receiver paper, and an image substantially identical to the writing develops on the receiver paper.
This image development is brought about by the contact of a colorless dye precursor or color former contained on the donor paper with a color developer contained on the receiver paper. Generally, a solution of the color former in a solvent is encapsulated in microcapsules and coated on the back side (CB) of the donor paper, which has an uncoated front or top side. The color formers, often termed "leuco dyes", are, for example, triphenyl methanes (such as Crystal Violet Lactone), xanthenes (such as N-102 flouran), and thiazines (such as Benzoyl Leuco Methylene Blue). The color developer is generally coated on the front side (CF) of the receiving paper. When the CB and CF sheets are placed in contiguous relationship and pressure, such as from a writing instrument or typewriter, is applied, the microcapsules are crushed. The released color former solution is then adsorbed onto the CF sheet, where it contacts the color developer. The image on the CF sheet develops from a chemical transformation of the color former to a colored form caused by the color developer. In general, this chemical transformation entails an acid-base type reaction wherein the color former changes from an uncharged, colorless state to a positive charged, colored state.
Alternatively, the color former/microcapsules and color developer can be coated onto the same paper sheet or the developer can be on the CB sheet and the color former/microcapsules can be on the CF sheet. For convenience hereafter, "CB sheet" refers to a paper sheet coated with the color former solution in microcapsules, and "CF sheet" refers to a paper sheet coated with a color developer. In addition, the manifold assembly of paired CB and CF sheets can include more than one pair of sheets.
Currently, two types of color developers are in wide use. Phenolic resins are polymeric long-chain compounds, which are generally of high molecular weight resulting from the polymerization of formaldehyde and a biphenol compound. Their use has several drawbacks, however. For example, they must be subjected to a long and complicated milling process to produce a resin of a specific particle size suitable for use in carbonless paper. In addition, the resins can decompose, releasing toxic formaldehyde. Next, the resins give a yellowish background to the CF sheet resulting in a higher image background for the writing image, which is accordingly of lower quality. Finally, the phenolic resins do not develop high image densities because the resins are of limited solubility in the color former solution again resulting in low image quality.
A second type of color developer is an acid-treated clay which has its own drawbacks. First, preparation of the clay requires careful treatment with a strong mineral acid because the color developing activity of an acid clay depends on the extent of hydrogen ion exchange into the clay to form color developing sites on the clay surface. In addition, a highly porous clay surface is necessary to ensure a good quality image which requires sufficient contact between the color former solution and the color developing sites on the clay surface. This problem of contact between the color former and acid clay is similar to the limited solubility of phenolic resins in the color former solution. Hence, image quality is highly sensitive to the chemistry and morphology of an acid clay, yet these parameters are very difficult to control.
In addition, CF sheets are prepared by coating a slurry of the color developer and an adhesive onto paper and letting the coating dry. The coating slurry is known in the art as a coating color. High solids concentrations, which includes solids content of the developer and the adhesive, in the coating color are desirable for good image quality, but concentrations above 45-50 weight % are generally unattainable with phenolic resins or acid clays due to the poor rheology, caused by high viscosity, of coating colors containing them.
Biphenolic compounds, which have not been polymerized like the phenolic resins, have been disclosed as color developers for certain lactone-type color formers in U.S. Pat. No. 3,244,550, "Manifold Sheets Coated with Lactone and Related Chromogenous Compounds and Reactive Phenolics and Method of Making", Farnham et al., issued Apr. 5, 1966. Farnham also discloses that "neutral or inert" pigments serve to make the coating containing the color developer whiter and more opaque. The pigments disclosed are titanium dioxide, zinc oxide, zinc sulfide or zirconium dioxide, which are all known as "hiding pigments" in the coating art, and have high refractive indexes of 2.0 to 2.7. Farnham does not disclose a non-hiding pigment such as calcium carbonate which has a refractive index of about 1.5. Next, Farnham uses an amount of pigment of about 1/10 of the coating weight on a dry basis. Farnham states the pigment has "no noticeable effect" on image development. Farnham also uses a large amount, at least 10 weight %, of the biphenol in the CF coating, and in the only operative example, the amount of phenol is 35 parts and the amount of pigment is 4 parts in a 100 part coating color.
U.S. Pat. No. 3,450,553, "Pressure Sensitive Record Material", Billet et al., issued June 17, 1969, discloses lignin as the color former in carbonless paper, and two specific phenols, phloroglucinol and 3-hydroxy phenyl urea, as color developers for lignin.
U.S. Pat. No. 4,349,218, "Copying Material Employing Flouran Color Formers", Garner, issued Sept. 14, 1982, also discloses phenolic color developers for flourans, such as 4-t-butylphenol, 4-phenylphenol, 2,2'-dihydroxydiphenyl, 4,4'-isopropylidenediphenol (Bisphenol A), and 4,4'-isopropyliden-bis-(2-methyl-phenol). Garner does not disclose a phenol admixed with a pigment such as calcium carbonate.
Gr. Britain Pat. No. 1,356,402, "Colour Developer Sheet for Pressure Sensitive Recording", issued June 12, 1974 discloses a color developer of "one or more clays capable of acting as color developers" and at least one monomeric phenolic compound. The amount of phenolic compound is about 1 to 10 parts by weight per 100 parts of clay. The clays listed are "attapulgite, acid clay, active clay, zeolite and bentonite." These clays are either the acid treated clays discussed above or certain smectite clays which are known as active color developers without acid treatment.
U.S. Pat. No. 4,046,941, "Support Sheet with Sensitized oating of Organic Acid substance and Organic High Molecular Compound Particulate Mixture", Saito, issued Sept. 6, 1977, discloses a two component color developer mixture of an aromatic carboxylic acid or its metal salt with an organic high molecular compound. The aromatic carboxylic acids disclosed include certain hydroxy carboxylic acids such as salicylic acid. The organic high molecular compound has a molecular weight above 400, such as polybutadiene, and must be compatible, i.e., miscible, with the aromatic acid. The amount of high molecular compound must also be enough to give the composition the desired resistance to heat, light and moisture. In one aspect, the mixture is adsorbed around an inorganic powder such as clay or calcium carbonate. Although Saito does disclose an admixture of CaCO.sub.3 and color developer, Saito's color developer must contain both the carboxylic acid compound and the high molecular compound. Moreover, the high molecular compound in the color developer presents the same drawbacks as phenolic resins.
U.S. Pat. No. 4,416,471, "Color-Developing Sheet for Pressure Sensitive Recording Sheet," Yamato, issued Nov. 22, 1983, discloses a color developing sheet with a layer of an organic color developer, calcium carbonate and a specific binder, an acrylamide modified polyvinyl alcohol, in an amount of 5-25 weight % to glue the developer and pigment on the sheet. This specific polyvinyl alcohol is critical to Yamato since he discloses that fine particle size calcium carbonate in a coating creates numerous gaps between particles which act as capillaries to adsorb the color former solution, and that this binder does not mask the gaps. Yamato discloses the organic color developer are phenolic resins or their metal salts, or organic acid substances such as a "metal salt of 2,2-bisphenol sulfone." Yamato does not disclose that his color developing sheet can be produced using a coating color of high solids content.
Although not directed to color developers, U.S. Pat. No. 3,894,168, "Paper Coating Pigment Material," Brockett, issued July 8, 1975, discloses a treated pigment color former. Brockett adsorbs a color former such as Crystal Violet Lactone on a pigment such as calcium carbonate to produce a coated pigment particle. However, the Brockett color former is unprotected from being converted to a colored form by, for example, air, the oil from human skin, or the other components in a paper coating.
What is needed is a low-cost, color-developer composition which is capable of developing high quality images. It is an object of Applicants' invention to provide such a composition comprising calcium carbonate. Another object is to use a color developing composition comprising a bisphenol. Another object is to provide a color color developer coating having high solids content. Other objects appear below.